Use memory_order_relaxed in EnqueueOrderGenerator::GenerateNext

As gab@ pointed out we don't really need the memory barrier here.

Bug: 852344
Change-Id: Iadff34dd8d571cdcc4323ce6dd0f1e757f4a0fd1
Reviewed-on: https://chromium-review.googlesource.com/1099163
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Greg Kraynov <kraynov@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567586}

base/task/sequence_manager/enqueue_order.h

@@ -45,7 +45,7 @@ class EnqueueOrder {
     // TODO(scheduler-dev): Is it the right atomic? https://crbug.com/852344.
     EnqueueOrder GenerateNext() {
       return EnqueueOrder(std::atomic_fetch_add_explicit(
-          &counter_, uint64_t(1), std::memory_order_seq_cst));
+          &counter_, uint64_t(1), std::memory_order_relaxed));
     }
 
    private: